The invention relates to a method for controlling an assist pump (a support pump) for fluid delivery systems with pulsatile pressure.
It is known, to arrange further pumps for the assistance of pulsatile working pumps in a fluid delivery system. Such assist pumps are normally driven at a constant number of revolutions. In correspondence to the differential pressure/volume flow—characteristic line of the assist pump, the assist pump reacts, when the input pressure is increased and the resulting change of the delivery height (difference of the input pressure to the output pressure), with an increase of the volume flow and vice versa. The volume flow through the assist pump falls, however, also in the low pressure phase, i.e. at a reduced input pressure, not to zero.
The steeper the differential pressure/volume flow—characteristic line of the pump is, the higher is the remaining volume flow in this phase. This can lead to the fact, that the volume flow stops and that the delivery system arranged in front of the assist pump, including the main pump, are acted upon by a negative pressure, which can lead to different disadvantages. E.g. the still flowing fluid can experience high turbulences, when flowing into the pump chamber of the main pump.
An especially sensitive fluid delivery system is the blood flow. Blood circulates, driven by rhythmical contraction of the heart, in a closed vascular system. During an error of the function of the heart, in the last few years blood flow assist pumps have been used, which should support a still present pulse of the heart. The blood is guided from the left heart chamber by means of bypassing the heart valve into the assist pump and from there into the aorta. Such assist pumps can be designed according to the displacement principal as pulsatile pumps and also according to the turbo principal as radial or axial flow machines. Pulsatile pumps according to the displacement principal have shown not to be advantageous because of the necessary expenditure for the synchronisation with the heart beat. In the pumps, working according to the turbo principal, the axial pumps are preferred, because of their small dimensions.
The known axial blood pumps consist essentially of an outer cylindrical tube, in which a delivery element, which is formed as a rotor of a motor stator arranged at the outside, rotates and moves the blood in axial direction. Furthermore, it is known, to magnetically support the rotor free of contact. Such an assist pump is known from WO 00/640 30.
If such an assist pump is driven with a constant number of revolutions, it results because of the above described conditions, that the blood is still delivered by the assist pump, when the heart chamber is in the decontraction phase.
The invention has the object to provide a method, by which the volume flow in an assist pump only acts supportingly in a simple way in the pressure phase of the main pump.